As wireless communication systems are being developed, the demand for improving speed thereof is increasing. In order to meet the demand, a wide frequency band needs to be used, but frequency resources are limited. As a result, as a method of transmitting more data while using the limited frequency band, a multi-input multi-output (MIMO) antenna technology is being used.
In the MIMO antenna technology, a precoding matrix selected in a codebook that is a set of a plurality of predetermined precoding matrices and a transmission signal vector are multiplexed and transmitted to a plurality of antennas. At this time, a receiving device selects a precoding matrix in the codebook on the basis of information on a state of a channel, and feeds back the selected precoding matrix to a transmitting device.
Meanwhile, a retransmission scheme of the wireless communication system includes a retransmission scheme in a medium access control (MAC) layer and a hybrid automatic retransmit request (HARQ) scheme integrating and using link performance of a physical layer. The HARQ scheme is a technology for recovering received data by combining retransmitted data with received information of previously transmitted data. In the HARQ scheme, the receiving device determines whether or not an error is generated in the reception signal by using, for example, a cyclic redundancy check (CRC) code. As the determination result, the receiving device transmits an acknowledge (ACK) message when no error is generated and transmits a negative acknowledge (NACK) message when an error is generated.
In the MIMO antenna technology, in the case of utilizing the codebook and adopting the HARQ, when the precoding matrix is selected in retransmission due to an initial transmission error in the same manner as the initial transmission, a diversity gain caused by the retransmission cannot be fully acquired. When the precoding matrix is selected in retransmission by utilizing reception information of the initial transmission, an optimum precoding matrix may be selected, but for this, information on the precoding matrix should be transmitted from the receiving device.
In order to maximize the retransmission diversity gain without receiving additional precoding matrix information from the receiving device, a set of the optimum precoding matrices needs to be generated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.